This invention relates generally to a video signal processing circuit and, in particular, is directed to a video signal processing circuit which mixes the information in successive lines of a video signal.
Commercial television systems use an interlaced scanning system consisting of frames each having a first, or odd field which reproduces alternate lines of the frame and a second, or even field interlaced with the first field and which reproduces the remaining lines of the frame. Each field consists of parallel television lines, the first of which begins in the upper left corner of the television screen and the last of which ends in the middle of the bottom of the screen. Each even field consists of parallel television lines interlaced between the lines of the odd field, and the first of which begins in the middle of the top of the television screen while the last line ends at the lower right corner of the television screen. The video at a point in a line from an even field, for example, contains video information which reproduces a part of the picture occurring just above or just below a corresponding point on the next adjacent line from the next successive odd field. The vertical distance between such corresponding points is one-half pitch, or one-half the center-to-center distance between adjacent lines of a field (which is equal to the vertical center-to-center distance between adjacent interlaced lines).
When the frames of the recorded video signals are reproduced out of their normal sequence, such as during a stop-motion, slow-motion, fast-motion, or reverse operation, an odd field will often be reproduced when an even field is expected, and vice-versa. Furthermore, when only even fields or only odd fields of the video signal are reproduced, or when a single field is repeatedly reproduced by a reproducing device from a video tape, a video sheet, or a video disc, direct use cannot be made of the reproduced synchronizing pulses since the video signal lacks the necessary relationship between vertical and horizontal synchronizing pulses for producing interlaced scanning. A locally generated synchronizing signal having the necessary relationships between horizontal (line) and vertical (field) synchronizing pulses is normally produced in the reproducing apparatus, and is substituted during reproduction for the corresponding recorded synchronizing pulses. The use of a locally-generated synchronizing signal, when a field is reproduced repeatedly, will result in the video signal being displayed on the television screen, and then being again displayed but shifted up or down by an amount corresponding to one-half a pitch. Accordingly, the reproduced picture appears to vibrate annoyingly up and down on the screen, even though the interlaced raster is itself stationary.
The shifting process described above occurs when processing either a monochrome television signal or a composite color television signal, but an additional problem arises when processing an NTSC composite color television signal by reason of the fact that the phase of the chrominance component in the NTSC composite color television signal has a specific and changing relationship from line to line, field to field, and frame to frame. Specifically, the phase of the color subcarrier is reversed from line to line and from field to field. The locally-generated synchronizing signal may contain a color burst signal having the correct changing phase relationship to the successive frames and fields for correctly demodulating a color signal in the normal reproducing mode. Nevertheless, when the same field is scanned repeatedly, as in the stop-motion mode, the phase of the reproduced chrominance signal will not always be in the relationship required by the NTSC system, and a correct color image cannot be reproduced. The above requirement of the changing relationship of phase is especially important when reproducing a composite color television signal for broadcasting.
One approach to solving the problem set forth above is disclosed in copending patent application Ser. No. 28,597, filed Apr. 9, 1979, and having a common assignee herewith. The signal processing circuit according to that copending application generates a reference synchronizing signal operative to define the various types of reference television fields (i.e., odd and even, and with reversed or unreversed color subcarrier phase) in a predetermined repeating order, and detects non-coincidence between at least one characteristic of the reproduced television field and one of the types of reference television fields. Reproduced video signals of two successive lines are mixed together to produce an interpolated signal and, if non-coincidence of the reproduced line synchronizing signal with the reference line synchronizing signal is detected, the interpolated television signal is applied to an output of the processing circuit. On the other hand, if coincidence of the reproduced with the reference line synchronizing signal is detected, the reproduced (i.e., uninterpolated) television signal is applied to the output of the processing circuit.
Such a video signal processing circuit performs satisfactory in many applications. However, its application is somewhat constrained when used with digitized video signals, such as those provided in a digital time base corrector, or those provided when a recorded digitized video signal is reproduced. If digitized video signals are to be processed in the above described video signal processing circuit, it is necessary that the circuitry for mixing together the two successive lines of the video signal be a digital adding circuit, and such digital adding circuits are normally quite complex. Furthermore, because such a video signal processing circuit requires interpolated and uninterpolated digitized video signals to be applied selectively to provide an output video signal having the proper sequence of frame types, a switch must be provided in addition to the digital adding circuit so as to further increase the circuit complexity.